The in-game information in this article is kept purely for historical purposes and may not need to remain under any other categories.

As of Patch 4.0.1, armor penetration has been removed from gear itemization, however it still exists in the game as a passive stat through the mastery system.[1]

Armor penetration (sometimes abbreviated ArP or called armor pen) is a value appearing on many talents and abilities that indicates the percentage of armor that will be ignored by an attack. ArP can be found as both Abilities (example: Sunder Armor), Talents like Mace Specialization or as Mastery bonuses. ArP is unique in that the more ArP you have, the greater the increase in damage becomes. Armor penetration is sometimes confused with spell penetration, even though they affect different things.

Until recently, this stat has not received much attention from Blizzard, however it was seen in the talent Serrated Blades since patch 1.10. Since itemization introduction of Arena Set 3, Blizzard has been popularizing the use of this modifier even into .

Contents

Armor penetration rating

Armor penetration is a rating system that affects a percentage of armor ignored. This means that upgrades scale more cleanly with all bosses but has a side effect of becoming less useful on low armor ones.

The conversion rate is about 7 ArP = 1 ArP rating. So if you had 1000 ArP, you now have 142.8 ArP rating. In the past, this would have been a 6% gain on a 15k armor foe and a 12.5% gain on an 8k armor foe; at level 80 it now gives a 9.2% gain to both targets.

ArP Rating required to ignore 1% armor

Level 60

Level 70

Level 80

3.75

5.92

13.99

Armor penetration rating against WotLK Raid bosses

The following tables provide detailed numbers for the effects of different levels of Armor Penetration Rating (ArP) in the case where a level 80 physical damage class attacks a level 83 raid boss with a given level of armor. ArP experiences diminishing returns for target armor lower than 8,317 armor.

Note: Due to various changes to armor penetration rating, these tables are outdated. The percent reduction and effective armor values are both correct; however, the armor penetration rating column uses the outdated 12.32:1 conversion (as opposed to the current 13.99:1 conversion).

In addition, what this entails for percentage based ArP is that the higher the armor the greater the ArP raw value and the lesser the armor the less ArP raw value. That is to say, percentage based ArP suffers diminishing returns for heavily sundered units in terms of raw ArP value, especially for those with very low armor value.

Now everything operates in percentages and stacks additively in two categories: Debuffs and personal Armor Penetration. The two categories stack multiplicatively with each other.

Armor penetration benefits

Armor penetration will increase non-Bleed physical damage by a percentage significant for virtually every target, however, for lightly armored targets it is less effective in the damage it provides.

From the perspective of a warrior, because armor penetration increases damage, he will get a moderate gain in rage generation as a result of higher armor penetration. Also, armor penetration causes the warrior to increase Execute damage.

Opportunistically, from a PvP point of view, a warrior may apply fewer stacks of Sunder Armor, assuming they have a higher armor pen than usual, because they have a head start on armor ignored. Or they may choose to boost the sustained damage further by a significant percentage by applying more stacks.

Technical explanation from Ghostcrawler

Quote from: Ghostcrawler

Okay, here is a fairly technical explanation we put together for how armor pen works.

We didn’t want Armor Penetration Rating to be too powerful against low armor targets, like it had been in BC. We also didn’t want Armor Penetration Rating to be too powerful against high armor targets.

So, we decided on a system where there is a cap on how much armor the Armor Penetration Rating can be applied to. So, the first X armor on the target is reduced by the percentage listed in the Armor Penetration Rating tooltip, and all armor past that X is unaffected. Another way of understanding that is we multiply the percentage in the tooltip times the minimum of the two values: the cap, and the amount of armor on the target after all other modifiers.

Computing the cap is a little tricky unless you are already familiar with how World of Warcraft armor works. There is an armor constant we’ll call C. C is derived as follows (in some pseudocode):

A level 80 warrior creature has 9729 armor. C=15232.5. So, the cap is (9729+15232.5)/3=8320.5. Let’s say a player has 30% armor penetration from armor penetration rating and no other modifiers that complicate the calculation (talents, Battle Stance, Sunder Armor, etc.). The game chooses the minimum of 8320.5 and 9729, so 8320.5. That is multiplied by 30% = 2496.15, and so that much armor is ignored. The effective armor on the target is 7232.85 (9729-2496.15). From a player point of view, the armor penetration rating didn’t ignore the full 30%, but instead ignored 25.66%. (85.5% as effective as expected).

These equations should help you be able to test and verify that Armor Penetration Rating is working correctly and as we designed. The tooltip is not actually inaccurate, as it states: “Enemy armor reduced by up to 30.00%.” That "up to" is key.

Please be sure to test without any other effects which modify the armor calculation (Battle Stance, Sunder Armor, Mace Specialization, etc.) as they may involve other systems that add additional complexity to the calculation.

The formula for the Armor Penetration cap can be simplified to ArPcap = (935/6)*x + y/3 -44335/6, where x is the target level and y the target armor. Then effArmor = targetArmor - ArP * min{ targetArmor, ArPcap} as explained above, with the Armor Penetration ArP of your character. ArP is given, lately changed in 3.1.2,[2] by the formula ArP = min{ 1,(Sum ArPcontributions)%}.

Calculating

Calculating the ARP it's done in 2 parts:

First up - the debuffs. These reduce your target's initial armor. For example, if your target has 10k armor and it gets 20% sunder, its armor would be 8k. The debuffs stack additively, so Faerie Fire will stack with sunder, totaling a 25% reduction on the target, taking it to 7.5k armor.

Second, comes the formula GC provided, you grab the armor after the debuff calculation and to that armor you apply the formula (armor + C)/3. Using the same example as before, lets say the target is level 80, so C=15232.5, now calculate (7500 + 15232.5)/3 = 7577.5. This makes the target's armor lower than the result, meaning that each 1% removes 75 armor and you are hitting a 0 armor target on the 100% ARP get. The ArP "buffs" (items, stances and such) stack additively too, meaning that 90% ARP on battle stance for a warrior would mean a 100% ARP final.

Patch changes

Patch 4.0.3 (15-Nov-2010): "We are always looking for ways to improve how player stats work. Our long-term plan is to simplify statistics and provide a more clear understanding of their effects on healing and damage dealing. For example, Spell Power will no longer be its own statistic, and Intellect will provide its benefits instead. We also plan to make Spirit the definitive mana-regeneration statistic, in which case we would remove the mana-per-5-seconds stat from gear. Confusing stats like defense and armor penetration will likely go away. Of course, existing items will be updated to reflect these changes. We're also adding a new stat called Mastery, which ties into a character's existing talents and will grant a unique function for each class. A rogue, for example, might gain an extra combo point through the Mastery stat." (Source)

Patch 4.0.3 (15-Nov-2010): Announced at the BlizzCon 2009 Game Systems Panel, armor penetration will no longer be available on gear. However, it will still be part of the game via the Mastery system. (Source)See the source link for an example of how armor penetration will be incorporated into talents in the Cataclysm expansion.

Patch 3.2.2 (2009-09-22): The amount of armor penetration gained per point of this rating has been reduced by 12%.